Abstract
In this paper, we examine the superconducting properties of MgB2+X thin films prepared with various compositions. MgB2+X thin films were prepared on polished silicon (100) single-crystal substrates using an electron beam evaporation technique without any post-annealing. The critical temperature of the MgB2+X thin film decreased in accordance with increasing B ratio in the B-rich composition region. However, ⊿Ms in the M-H curves of the B-rich MgB2 thin films were larger than that of the stoichiometric MgB2 thin film and the B-rich composition resulted in a longer c-axis length for the MgB2 crystal. These results suggest that Mg vacancies or B substitution on Mg sites in the MgB2+X crystals induced by the Mg-deficient composition act as effective pinning centers. Moreover, the chemical composition of the MgB2 superconductor strongly affected the stability of the microstructure in the MgB2 thin film. Therefore, we conclude that the slightly B-rich composition is preferable to the stoichiometric composition for the MgB2 superconductor.
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